1. Field of the Invention
The present invention relates to a piezoelectric actuator, a droplet ejection apparatus, and a manufacturing method thereof, and more particularly to a piezoelectric actuator that is subjected to polarizing treatment and is displaced as a result of the application of a drive electric field, to a droplet ejection apparatus that ejects droplets by using the displacement of the piezoelectric actuator, and to a manufacturing method thereof.
2. Description of the Related Art
There are known inkjet heads having multiple nozzles (holes) and forming images on paper or another such recording medium by ejecting ink droplets from the nozzles onto the recording medium while moving relatively to the recording medium.
Examples of such inkjet heads include inkjet heads designed so that ink is supplied to pressure chambers partially configured from a diaphragm, the diaphragm is deformed by driving of piezoelectric actuators by the application of electrical signals corresponding to image data onto the piezoelectric actuators, the capacity of the pressure chambers are thereby reduced, and the ink inside the pressure chambers are then ejected as droplets from the nozzles.
Examples of the piezoelectric actuators used in such inkjet heads generally include so-called 31-mode actuators, which use the displacement (strain) induced in the direction perpendicular to the poling direction when the drive electric field is applied in the poling direction; so-called 33-mode actuators, which use the displacement induced in the poling direction when the drive electric field is applied in the poling direction; and so-called 15-mode actuators, to which the drive electric field is applied in the direction perpendicular to the poling direction.
An example of a piezoelectric actuator in the related art is shown in FIG. 13. In FIG. 13, the piezoelectric actuator 960 is a 31-mode actuator, and has a plate-shaped piezoelectric body 961, and a pair of entire surface electrodes 962 and 963, which are disposed on both sides of the piezoelectric body 961 to face to each other across the piezoelectric body 961 in the thickness direction. The entire piezoelectric body 961 is uniformly polarized in the thickness direction by the entire surface electrodes 962 and 963, as indicated by the arrows in FIG. 13. The piezoelectric actuator 960 thereby polarized is attached to a diaphragm 56. Specific voltages corresponding to image data relating to image formation are then applied to the pair of entire surface electrodes 962 and 963, and stress is applied to the diaphragm 56 in the thickness direction using the displacement induced in the piezoelectric body 961 in the direction perpendicular to the thickness direction, as shown in FIG. 14. FIG. 15 shows a schematic view of the shape of the diaphragm 56 when displacement is at a maximum.
Japanese Patent Application Publication No. 10-119262 (see FIG. 1 in particular) discloses a 33-mode driven piezoelectric actuator. This actuator has a sheet-shaped piezoelectric body, and a total of two pairs of drive electrodes; the drive electrodes of one of the pairs are disposed on both sides of the piezoelectric body at positions corresponding to the middle of the pressure chamber or cavity (positions over the pressure chamber), and the drive electrodes of the other of the pairs are disposed on both sides of the piezoelectric body at positions corresponding to the side wall (positions diagonally above the pressure chamber). When the piezoelectric body is polarized, the pair of electrodes at the middle is used as the positive and the pair of electrodes at the side wall is used as the negative, and the piezoelectric body is polarized in the direction along its surface or the direction perpendicular to the thickness direction. The drive electric field is applied during driving in the same direction as the polarization.
Piezoelectric actuators with various structures such as those described below have been proposed with the purpose of achieving high displacement.
Japanese Patent Application Publication No. 2003-008091 (see FIG. 1 is particular) discloses an actuator having a sheet-shaped piezoelectric body, with pairs of drive electrodes (the lower electrodes are the common electrode) disposed on both sides of the piezoelectric body at positions corresponding to the middle of the pressure chamber or cavity, and at positions corresponding to the peripheral edge of the pressure chamber. The piezoelectric actuator is designed so that strains oriented in two ways opposite by 180 degrees are created in the middle portion and the peripheral edge portion of the piezoelectric body as a result of applying opposite voltages in the drive electrodes in the middle and the drive electrodes in the peripheral edge, respectively. The poling directions are set in advance also to be oriented in two ways opposite by 180 degrees in the middle portion and the peripheral edge portion of the piezoelectric body, respectively.
Japanese Patent Application Publication No. 2002-355981 (see FIGS. 1, 2, and 5 in particular) discloses a 15-mode driven piezoelectric actuator, in which the direction of the drive electric field is substantially perpendicular to the poling direction, and the piezoelectric actuator has a layered structure in which a plurality of thin plates of piezoelectric ceramic are stacked. A plurality of dedicated polarizing inner-layer electrodes are formed in the interior of this layered structure, these dedicated polarizing inner-layer electrodes are used to achieve polarization in the direction along the surface or the direction perpendicular to the thickness direction, and the dedicated polarizing inner-layer electrodes are then removed by polishing or the like. Thereafter, a plurality of dedicated driving electrodes are formed, and an electric field is applied in the thickness direction through these dedicated driving electrodes. In this piezoelectric actuator, the polarization is achieved at the directions somewhat oblique to the direction along the surface or the direction perpendicular to the thickness direction on a microscopic level as a result of the layered structure, and the drive electric field is applied in the direction somewhat oblique to the poling direction. However, the piezoelectric actuator remains the 15-mode actuator and is polarized in the direction along the surface or the direction substantially perpendicular to the thickness direction as described above, and a drive electric field is applied in the thickness direction so as to be substantially perpendicular to the poling direction.
Japanese Patent Application Publication No. 2002-368297 (see FIG. 1 in particular) also discloses a 15-mode driven piezoelectric actuator with a layered structure. A plurality of dedicated polarizing inner-layer electrodes are formed in the interior of this layered structure, the dedicated polarizing inner-layer electrodes are connected to lead electrodes on the surface of the piezoelectric body by means of a through-hole structure prior to the polarization treatment, the connection is severed after the polarization treatment, and the polarizing electrodes themselves remain in the layered structure while the dedicated driving electrodes are used during driving.
Japanese Patent Application Publication No. 2003-224312 (see FIG. 8 in particular) discloses a piezoelectric actuator with a layered structure, in which a plurality of inner-layer electrodes are formed in the interior of the layered structure, and the actuator has a middle portion (first region) wherein 33-mode driving is performed in which the polarization is achieved in the thickness direction and the drive electric field is applied in the poling direction (the thickness direction) to induce the displacement in the poling direction (the thickness direction), and a peripheral portion (second region) wherein 15-mode driving is performed in which the polarization is achieved in the thickness direction and the drive electric field is applied in the direction perpendicular to the poling direction (the thickness direction).
Japanese Patent Application Publication No. 2002-292869 (see FIG. 4 in particular) discloses a piezoelectric actuator with a layered structure, in which a plurality of inner-layer electrodes are formed in the interior of the layered structure, and the actuator has a top layer wherein 33-mode driving is performed in which the polarization is achieved in the direction perpendicular to the thickness direction and the drive electric field is applied in the poling direction (perpendicular to the thickness direction) to utilize the displacement in the poling direction (perpendicular to the thickness direction), and a bottom layer wherein 31-mode driving is performed in which the polarization is achieved in the thickness direction and the drive electric field is applied in the poling direction (the thickness direction) to utilize the displacement in the direction perpendicular to the poling direction (the thickness direction).
However, the above-described techniques in the related art have had problems in that either high displacement cannot be achieved, or manufacturing costs are increased in order to achieve high displacement.
The actuator disclosed in Japanese Patent Application Publication No. 10-119262 has the two pairs of drive electrodes disposed along the surface or in the direction perpendicular to the thickness direction, and the amount of displacement is always less at the same voltage than with the actuator having the drive electrodes that are disposed on both sides of the piezoelectric body, as in the piezoelectric actuator 960 shown in FIG. 13. The drive electric field must be increased to achieve a displacement equal to or greater than that of the piezoelectric actuator in FIG. 13, which results in an inevitable increase in power consumption.
In the actuator disclosed in Japanese Patent Application Publication No. 2003-008091, since the poling directions must be oriented in two ways opposite by 180 degrees in the middle portion and the peripheral edge portion of the piezoelectric body, then generally the polarization treatment must be conducted twice, requiring more labor during manufacture, which leads to an inevitable increase in manufacturing costs.
In the actuator disclosed in Japanese Patent Application Publication No. 2002-355981, the dedicated polarizing inner-layer electrodes separate from the drive electrodes must be formed inside the layered structure, and the dedicated polarizing inner-layer electrodes must be removed by polishing or the like after the polarization treatment, requiring more labor during manufacture, which leads to an inevitable increase in manufacturing costs.
In the actuator disclosed in Japanese Patent Application Publication No. 2002-368297, the dedicated polarizing inner-layer electrodes separate from the drive electrodes must be formed inside the layered structure, and the connection between the dedicated polarizing inner-layer electrodes and the lead electrodes on the surface of the piezoelectric body must be severed after the polarization treatment, requiring more labor during manufacture, which leads to an inevitable increase in manufacturing costs.
In the actuators disclosed in Japanese Patent Application Publication Nos. 2003-224312 and 2002-292869, since the drive electric fields are applied in the plurality of modes, then there is a greater number of electrodes, the positional relationship between the electrodes becomes complicated, and errors in the positions thereof affect displacement. Therefore, individual differences between piezoelectric actuators are easily caused by nonuniformities in the electrode arrangements, and efforts to maintain consistency between the piezoelectric actuators reduce the yield rate, leading to an inevitable increase in manufacturing costs.
Moreover, the actuators disclosed in Japanese Patent Application Publication Nos. 2002-355981, 2002-368297, 2003-224312, and 2002-292869 must have the layered structure, and therefore inevitably have higher manufacturing costs than a piezoelectric actuator that can be configured as a single plate, such as is shown in FIG. 13. Another problem with the actuators in the related art is that it is difficult for them to be thinned.
Next, the matter of errors in aligning positions during mounting of the actuator 960 in the related art shown in FIG. 13 is described.
FIGS. 16A and 16B are schematic diagrams showing the relationship between the positional relationship of the piezoelectric actuator 960 to the pressure chamber 52 and the amount of stress-induced displacement in the thickness direction. In FIGS. 16A and 16B, the piezoelectric actuator 960 is not shown for the sake of simplicity, and the positional relationship is depicted by the vertical line A52 passing through the middle of the pressure chamber 52 and the vertical line A960 passing through the middle of the piezoelectric actuator 960. FIG. 16A shows the state of displacement of the diaphragm 56 in the thickness direction at the most preferred positional alignment, in which the vertical lines A52 and A960 coincide. FIG. 16B shows the state of displacement of the diaphragm 56 in the thickness direction at a positional alignment in which the vertical lines A52 and A960 are misaligned.
As shown in FIGS. 16A and 16B, when the vertical lines A52 and A960 are misaligned, the amount of displacement of the diaphragm 56 in the thickness direction decreases according to the amount of misalignment Agap. More specifically, the difference in the amounts of displacement of the diaphragm 56 in the thickness direction (h90-h91) increases as the amount of misalignment Agap increases. In other words, the piezoelectric actuators 960 in the related art have had problems in that there are likely to be performance differences between the actually produced individual piezoelectric actuators 960 as a result of errors in positional alignment, and the yield rate decreases as a result of such performance differences, which leads to an increase in manufacturing costs.